As is well known in the industry, many wells require “stimulation” in order to promote the recovery of hydrocarbons from the production well. This stimulation is commonly done via a method called “fracturing” or “fracking”, where the propagation of fractures is stimulated via the down-hole injection of a pressurized fluid. To keep this fracture open after the injection stops, a solid proppant, commonly a sieved round sand, is usually added to the fluid.
The injected fracking fluid is typically water. Chemical additives are applied to tailor the injected material to the specific geological situation, protect the well, and improve its operation, though the injected fluid is approximately 98-99.5% percent water.
Water as a fracturing fluid has many drawbacks. First, large quantities of water are needed, hydraulic fracturing typically requires millions of gallons of water per well. Usually this water needs to be trucked in and after fracturing some of the injected water returns to the surface once the well starts production. This produced water will be contaminated and needs to be treated and disposed. Supply and disposal of such large quantities of water is costly and also increases the environmental impact of fracturing if only because of the large number of trucks that are involved with water supply and disposal.
Secondly, water, once injected into the pores, can be difficult to remove and/or causes swelling, blocking the pores, reducing the hydrocarbon productivity. Water can also react with the minerals, the salts and native water and hydrocarbons down-hole, resulting in e.g. reservoir contamination. Cleaning up the well after hydraulic fracturing often requires flaring of early production.
The use of a liquefied petroleum gas (LPG), commonly a mixture of propane and butane and minor other components, as a fracturing fluid does not have the same drawbacks as water. The preparation and use of LPG laden with gelled proppant as a fracturing fluid is described e.g. in US 2010/0051272, CA 2639539, US 2012/0000660 and US 2005/0189112. LPG has a lower density than water; hence less tonnage needs to be trucked in for the same volume of fracturing liquid. LPG is a liquid when pressurized and injected into a reservoir, as long as the temperature of the injected fluid stays below its critical temperature. When the pressure is released, the LPG will vaporize and/or blend with the native hydrocarbons, hence no residual fracturing liquid will remain in the pores.
Unlike water, LPG will also not react down-hole and/or cause swelling and blocking of the pores in the reservoir. The vaporized LPG will stay in the reservoir until it is evacuated together with the native hydrocarbons and brought to market usually via a pipeline. As such there is no need for separate costly disposal of the fracturing fluid. However, the key drawback of using LPG as fracturing fluid is the relatively high cost of the LPG. A further drawback of LPG is that it changes the heat value as well as other important quality specifications of the product gas. This is especially a concern as LPG will not have sufficient time to diffuse through the reservoir and homogeneously mix with the product gas. As a consequence, the composition of the produced gas will fluctuate over time as the propane and butane concentration will be higher in the initial stages of production and decrease over the lifetime of the field. Quality fluctuations are likely to result in suboptimal downstream gas processing and/or combustion, for example an LPG extraction plant would need to be over-designed in order to handle the maximum LPG concentration during early production stages. Burners or engines running on the gas might need to be readjusted as the quality of the product gas changes.
U.S. Pat. No. 3,822,747 discloses the use of liquefied gas, in particular liquefied nitrogen, as a fracturing fluid, which is allowed to vaporize in the formation, thus fracturing the formation. Liquefied methane is mentioned as an alternative to liquid nitrogen for use in reservoirs of higher oil viscosity.
US 2010/0101796 teaches the use of nitrogen combined with a proppant supported by a viscosified liquid hydrocarbon and a viscosity breaker for fracturing subterranean formations.
US 2006/0065400 proposes to use liquefied natural gas (LNG) as a fracturing fluid. Proppants may be blended in prior to pumping the LNG into the well.
This invention addresses the drawbacks of LPG fracturing while still retaining most if not all of the advantages of LPG fracturing.